在NMR中，為何當外加射頻場的頻率與原子核自旋進動的頻率相同的時候，射頻場的能量才能夠有效地被原子核吸收，為能級躍遷提供助力，而其他頻率則不能令使能級躍遷?

This is, in fact, how resonance works. Just like that occurs in mechanical system, when the frequency of a driving system equals to the natural frequency of a vibrating system, energy will be delivered from the driving system to the vibrating system, resulting in an enhancement of the vibration.

In NMR, the atoms are under a steady magnetic field. The nucleii precess about the magnetic lines of force with a frequency particular to that type of nucleii. The external radio frequency acts as the driving frequency. When the driving frequency and the precession frequency are the same , resonance occurs, and energy is delivered from the external radio wave to the nucleii. The energy levels of the nucleii are thus enhanced.